The introduction of high activity Ziegler-Natta catalyst systems has lead to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefin polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
Recently, gas fluidized bed reactors have been utilized to produce so called sticky polymers.
The term "sticky polymer" is defined as a polymer, which, although particulate at temperatures below the sticking or softening temperature, agglomerates at temperatures above the sticking or softening temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to excessive agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into agglomerates of much larger size than the original particles. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by refluidizating the polymer by passing the fluidizing gas passing through the distributor plate is insufficient to break up the agglomerates which form and the bed will not refluidize. These polymers are classified as those which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the resin, it is predominantly governed by the temperature and the crystallinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline products such as very low density polyethylene (VLDPE), ethylene/propylene polymethylene (EPM), ethylene/propylene diene polymethylene (EPDM) and polypropylene (PP) copolymers usually display a larger tendency to agglomerate to form larger particles.
Because of the stickiness of these resins, the art has been producing these resins at temperatures below the softening temperatures of the polymers. More recently, however, U.S. Pat. No. 4,994,534 discloses a process for producing sticky polymers at polymerization reaction temperatures in excess of the softening temperature of said sticky polymers in a fluidized bed reactor in the presence of a catalyst, which comprises conducting the polymerization reaction above the softening temperatures of the sticky polymers in the presence of about 0.3 to about 80 weight percent, preferably about 5% to about 75% based on the weight of the final product of an inert particulate material having a mean particle size of from about 0.01 to about 10 micrometers or microns.
In effect, these inert particulate materials act as fluidization aids which prevent the sticky polymers from agglomerating and fouling the reactor walls. In order to insure long-term reactor operability, an excess of inert particulate material must be maintained in the reactor. "Excess" means more inert particulate material than will be bonded to the polymer. The concentration of excess inert particulate material on the polymer particles may range from 0.1 to 30 wt %.
As mentioned in the patent, inert particulate materials that might be employed include carbon blacks, silicas, clays and other like materials. The particle size of the inert particulate material ranges from sub-micron to tens of microns.
Extra attention must be paid to the handling of resin leaving the reactor if it contains appreciable amount of free inert particulate material. In particular, processing steps involving mechanical agitation and especially fluidization cause release of the free fine material. Filtration systems requiring blowback must be employed. Upsets requiring opening of the downstream process system will have an adverse environmental impact.
In addition, the free inert particulate material also contains large amounts of adsorbed reaction components. In particular, large quantities of diene monomer, e.g. ethylidene norbornene (ENB) and aluminum alkyl are adsorbed. These materials have an adverse effect (odor, reduced cure) on the product resin. The extra inert particulate material itself may reduce the marketability of the product.
In addition, more inert particulate material, diene monomer, and aluminum alkyl must be added to the reactor to replace that removed with the free inert particulate material. For example when utilizing carbon black, the amount of free carbon black on resin leaving the product discharge tank is frequently about 5 wt %. Typical loadings of diene monomer, e.g. ENB and aluminum alkyl on the carbon black are 2 and 0.5 wt % respectively.
It would therefore be extremely beneficial to provide a process for recycling inert particulate material used in sticky polymer gas phase polymerization reactors.